1. Field of the Invention
The invention relates to gas detection systems utilizing non-radioactive detectors.
In particular, the invention relates to gas detection systems utilizing a gas sensor whose elements are exposed to the atmosphere such that the resistance between such elements varies with the concentration of selected gases contained within the atmosphere.
Specifically, the invention relates to gas detection systems utilizing a variable resistance gas sensor having detection elements which are heated to proper operating temperatures and which sensor inherently provides false alarm indications when first activated and is further susceptible to staccato false alarm indications due to slight inherent drift operating characteristics in the course of its utilization, the invention providing the means for inhibiting such false indications of gas detection.
2. Prior Art
On July 4, 1922, U.S. Pat. No. 1,421,720 was issued to Roberts for his method of and apparatus for detecting the presence of one gas in another. Roberts utilized a gas sensor containing a pair of electrodes, at least one of which was heated. The electrodes were exposed to the atmosphere and current passing from one electrode to another was dependent upon the amount of gas to be detected contained within the atmosphere impinging upon the electrodes.
An ozone gas detector invented by Webster et al was disclosed in U.S. Pat. No. 3,778,229, issued Dec. 11, 1973. Webster et al provided a tubular substrate of high electrical resistivity and coated this substrate with a metal oxide whose resistance increased significantly with increasing ozone concentration in the atmosphere. Electrodes were provided as means for measuring the varying electrical resistivity of the oxide coating. An electrically heated filament was utilized to raise the oxide coating to the operating temperature to produce a significant change in the electrical resistance of the metallic oxide coating in response to variations in ozone concentration in the atmosphere.
Typical of the sensors employed by Roberts and Webster et al was the fact that a non-stable gas sensing element was provided while the sensor was brought to its effective operating temperature.
Apparatus for determining the alcoholic content in the blood as a result of sampling a person's breath utilizing a variable resistance gas sensor of the type manufactured by Figaro Manufacturing Company, Osaka, Japan is disclosed in U.S. Pat. No. 4,177,668 to Holmberg, issued Dec. 11, 1979. A Figaro TGS 812 gas sensor provides one element of a Wheatstone bridge. When the system is activated and the sensor is brought to operating temperature, the bridge is balanced by means of a potentiometer. The presence of gas, detected by the gas sensor, results in an unbalance in the bridge which is, in turn, detected by a voltage comparator which controls an alarm which is activated upon the detection of gas in the atmosphere. Because of the inherent instability of the Figaro gas sensor as it is brought up to operating temperature, Holmberg provides a manually operated circuit to prevent full activation of the system until the sensor has reached operating temperature. This circuit inhibits the production of false alarms while the sensor is brought to operating temperature and stabilized. Full activation of the system is thereafter achieved by manual closing of a switch following the illumination of a ready light indicating that the apparatus is ready to be used and the sensor has reached its operating temperature.
It is seen that gas detection systems utilizing thermally heated gas sensors are well known in the prior art and that all such systems are unstable immediately after activation and remain unstable until the temperature of the gas sensor has been raised and established at its operating point. Those skilled in the art of thermal sensitive devices will be aware of the inherent small drifting variation in performance characteristics of such devices provoked by thermal drift about their selected operating temperature. Such inherent initial unstability of the gas sensor requires that full implementation of a gas detection system utilizing such sensors be delayed until the gas sensor has stabilized at its operating temperature. In the gas detection systems of which the inventor has knowledge such full implementation of the gas detection system is achieved by manual control of the operator after the sensor has stabilized. In systems utilizing voltage comparators to control a sensible alarm, "chattering" of the alarm, that is, short staccato alarm indications, will be produced frequently due to the inherent drift in operating characteristics of the sensor during the period of time the alarm system is activated.
It is an objective of the invention to provide a filament heated, variable resistance gas sensor gas detecting system which avoids the production of false alarms following initial system activation without monitoring or further activity on the part of the operator.
It is a further objective of the invention to provide such a gas detection system which is inherently free of production of staccato false gas detection indications produced by thermal drift performance characteristic changes within the gas sensor.